In the formation of silver halide emulsions for use in photographic materials, a desirable characteristic is to have photographic materials that are capable of being rapidly processed. It is known that silver halide emulsions having high levels of chloride content (greater than 90 mole % chloride) are especially useful in achieving rapid processability due to their higher solubility compared to emulsions having greater than some minimum levels of other halides (for example, emulsions having greater than 10 mole % bromide or more than 3-5 mole % iodide or both).
Silver halide emulsions including those high in chloride content are also known to require some form of chemical sensitization in order to increase their photographic efficiency. Chemical sensitization of an emulsion involves the addition of one or more chemical sensitizing agents where the sensitizing agent is capable of undergoing a chemical reaction on the silver halide grain surface during the application of thermal energy for some time period. The chemical sensitization involving adding chemical sensitizer to an emulsion and heating is often referred to as chemical digestion of the silver halide emulsion. Emulsions, especially high chloride content emulsions also require spectral sensitization involving the addition of surface adsorbing sensitizing dyes to the emulsion grains which make the grains sensitive to specific wavelengths of light. In addition, silver halide emulsion grains are often treated with other photographically useful chemical compounds such as salts of other halides which can cause surface conversion of the host emulsion grains to a mixed halide composition. Other known photographically useful compounds which can be added are antifoggants, stabilizers, metal dopants, silver halide solvents, ripeners, supersensitizers, coating aids and surfactants. These photographically useful compounds can be added prior to, during, or after the chemical sensitization or the spectral sensitization steps.
It is known that silver halide emulsions, including those high in chloride content, often exhibit substantial reciprocity law failure which can limit their usefulness. That is, say these emulsions can exhibit a large change in photographic sensitivity and gradation due to a change in the illuminance of exposure. It is also known to incorporate certain metal dopant compounds to the silver halide emulsion grains in order to improve the reciprocity performance of these emulsions. It is further known that in order for the metal dopant to be effective in modifying silver halide emulsion performance such as reciprocity law failure, the metal dopant must be incorporated into the silver halide crystal which can be accomplished either during grain formation or during the sensitization process. It is also known that if the metal dopant is to be added after the emulsion grain formation as part of the finishing, it is necessary to perform a surface halide conversion to cause the dopant to be incorporated.
The terms photographic "sensitivity" and "speed" are used interchangeably herein, as are "gradation" and "contrast".
It has been disclosed in U.S. Pat. No. 5,284,745 --Ohshima and in U.S. Pat. No. 5,391,474 --Haefner et al that high silver bromide content localized phases may be formed on high silver chloride content host emulsions by the use of fine grain silver bromide emulsions having iridium incorporated therein.
U.S. Ser. No. 08/390,450 filed Feb. 17, 1995, now U.S. Pat. No. 5,523,200, with inventors Hahm et al also discloses that iridium, as well as other materials, may be added by the use of Lippmann emulsions.